1. Field of the Invention
This invention pertains to an electronic device containing a composite substrate which is a multi-layer of which at least one layer is polycrystalline, and to a method for making same.
2. Description of Related Art
A large area, inexpensive substrate for the growth of epitaxial layers gallium nitride (GaN) and silicon carbide (SiC) has been a long-sought goal. Currently, single crystal GaN and SiC substrates are the predominantly used substrates for epitaxial growth. However, single crystal SiC substrates are very expensive and are currently available in small substrate sizes. For instance, 8xe2x80x3 polycrystalline SiC substrates are available for about $800 whereas the largest single-crystal SiC substrate available is 2xe2x80x3 and is available for about $1000; large diameter microwave insulating substrates such as about $100 for a 4xe2x80x3 aluminum nitride (AlN) substrate, can be obtained whereas microwave insulating single-crystal substrates are expensive, costing about $4600 for 2xe2x80x3 diameter SiC substrate.
An alternative approach that has been investigated for SiC epitaxial growth on a large area substrate has been the growth of the cubic polytype of SiC (also referred to as the 3C or beta polytype of SiC) on a silicon substrate. The 3C polytype of SiC is desirable for its high electron mobility and high breakdown field for power electronic device applications, and its isotropic mobility characteristics for sensor applications. However, the large lattice mismatch (xcx9c20%) and thermal expansion mismatch (xcx9c8%) between SiC and silicon have to date prevented the growth of high quality SiC epitaxial layers on silicon substrates. An additional problem with this approach is that the optimum growth temperature for SiC epitaxial growth is between 1500xc2x0 C. and 1600xc2x0 C., well above the 1350xc2x0 C. maximum use and the 1450xc2x0 C. melting temperature of a silicon substrate.
Another approach for 3C-SiC growth on silicon substrate has been to first carbonize the silicon surface forming a thin 3C-SiC layer and then to grow 3C-SiC epitaxial layers on the carbonized silicon surface at a growth temperature below 1350xc2x0 C. There is a tendency for anti-phase domains to form in the epitaxial layer for 3C-SiC growth on a (100) orientation silicon substrate. More recent studies have included the growth of 3C-SiC on silicon-on-insulator (SOI) substrates.
Wide bandgap material has recently been demonstrated to be very beneficial for microwave power transistor applications, and for blue-green laser and light emitting diodes. GaN epitaxial layers have typically been grown on a sapphire substrate or on single crystal SiC substrates. There are continuing searches for new substrates for GaN growth. Sapphire is electrically insulating, a disadvantage for vertical current conducting optical emitters and power devices, and has relatively high thermal impedence which is a disadvantage for high power microwave devices. The best quality GaN epitaxial layers have been obtained for material grown on SiC substrates, however, single crystal SiC substrates are very expensive and are only available in small substrate sizes. GaN epitaxial growth on silicon substrates is recently being investigated as an approach to obtain GaN epitaxial growth on large area substrates. There is however, significant thermal expansion mismatch between GaN and silicon which leads to cracking of the epitaxial layer for thick GaN epitaxial layers. The silicon substrate is not suitable for microwave applications because of the microwave loss in the conducting silicon substrate. There is also a significant lattice mismatch between GaN lattice and silicon lattice, however, GaN epitaxial layers with reasonable electron mobilities have been grown on a silicon substrate.
For GaN growth on a silicon substrate, different poly-types of GaN have a tendency to form, depending on the orientation of silicon substrate. Typically, cubic poly-types of GaN will form on a (100) orientation silicon substrate. Likewise, hexagonal poly-types of GaN will form on a (111) orientation silicon substrate. In some cases, a preferred method to grow GaN on silicon is to first form a thin layer of cubic-SiC on the silicon surface by carbonization prior to the grown of GaN. There is a relatively good lattice constant match between cubic-GaN and cubic -SiC, and also between hexagonal-GaN and hexagonal-SiC. Care should be taken in the GaN growth process, to avoid the formation of silicon nitride on the silicon surface prior to the GaN growth.
Non-single crystal ceramic substrates can be designed to have optimized mechanical, thermal expansion, thermal conduction, or electrical conduction properties for particular applications. One polycrystalline ceramic substrate that has especially desirable properties is poly-SiC. Poly-SiC substrates are manufactured commercially in hot pressed sintered form, reaction bonded form, and chemical vapor deposited (CVD) form. The CVD poly-SiC substrates are available commercially in substrate size up to 200 mm diameter, with thermal conductivity as high as 310 W/mK, electrical resistivity as high as 100,000 ohm-cm at room temperature, electrical impedance as low as 0.01 ohm-cm, maximum use temperature greater than 2000xc2x0 C., and excellent thermal expansion matching to single crystal cubic-SiC. Hot pressed sintered poly-SiC substrates are commercially available that have many of the above characteristics. Ceramic AlN substrates are available commercially in substrate sizes of 100 mm diameter, with thermal impedances as high as 190 W/mK, electrical resistivity as high as 1013 ohm-cm at room temperature, and excellent thermal expansion match to single-crystal GaN. Polycrystalline diamond substrates have thermal conductivity greater than 1000 W/mK. Ceramic silicon nitride has good thermal expansion matching to silicon. Ceramic graphite substrates are available with electrical impedances as small as 0.001 ohm-cm at room temperature. AlSiC substrates are commercially available, and have good expansion matching to silicon.
Direct wafer bonding typically requires polishing that the surfaces of the substrates to be bonded to a root mean square (RMS) surface roughness of less than 1 nm. Materials such as silicon, GaAs, and sapphire can readily be polished to a surface roughness of less than 1 nm RMS. Extensive polishing is required for hard materials such as silicon carbide and diamond to achieve this surface roughness condition. There are a number of approaches that can be used to the bonding of two substrates to reduce the requirement that the two substrates. One approach is to deposit a material such as polysilicon, silicon dioxide, silicon nitride, or metal on the substrate surface, and then polish the material to a surface roughness of less than 1 nm RMS. The use of pressure, temperature, or vacuum separately or in combination also reduces the requirement to have a surface polishing of 1 nm or less. If one of the substrates is thin, then the thin substrate will more easily conform to the other substrate during bonding and thus reduce the requirement for surface roughness less than 1 nm RMS.
Accordingly, it is an object of this invention to provide an improved electronic device and a method of growth of at least one single-crystal material layer that is wafer bonded to a composite substrate with the mechanical, thermal expansion, thermal conduction, electrical conduction, and optical transmission properties of the substrate selected to optimize the epitaxial growth of a single-crystal material layers, and to optimize performance of electronic devices formed using the grown epitaxial material layer.
Another object of this invention to provide improved electronic devices that are fabricated in a single-crystal material layer that is wafer bonded to a composite substrate with the mechanical, thermal expansion, thermal conduction, electrical conduction, and optical transmission properties of the composite substrate selected to optimize the performance of the electronic devices.
Another objective of this invention is to encapsulate one or more layers of a composite polycrystalline substrate with a thin chemical vapor deposition layer to ensure that impurities that are within in the ceramic polycrystalline substrate don not diffuse out of the ceramic polycrystalline during the fabrication of the electronic device.
Another object of this invention is to thin the composite substrate from the backside after the electronic device has been fabricated to substantially or entirely remove one layer of the multilayer composite substrate.
Another object of this invention is the provision of a via from the backside after the electronic device has been fabricated with the via made substantially or entirely through one layer of the multilayer composite substrate, and then the via partially or entirely filled with a thermally conductive material layer such as diamond or copper.
Another object of this invention is to provide wide bandgap epitaxial material layers on optimized large diameter composite substrates for a wide number of applications, including lateral conducting microwave power devices, vertical conducting microwave power devices, lateral conducting power switching devices, vertical conducting power switching devices, and vertical and lateral conducting optical laser and LED emitters.
Another object of this invention is to provide a method to grow epitaxial layers of cubic-polytype of GaN and hexagonal-polytype of GaN on a thin single crystal layer of silicon or GaAs that is wafer bonded to a composite substrate.
Another object of this invention is to provide a method to grow a hexagonal or cubic polytype GaN epitaxial layer on a thin compliant single-crystal layer that is wafer bonded to composite substrate for a wide range of applications, including lateral conducting microwave power devices, vertical conducting microwave power devices, and vertical conducting optical laser and light emitting diodes (LEDs).
Another object of this invention is to provide a method to fabricate a material structure where there is an additional insulating or electrically conducting wafer bonded material layer between the thin wafer bonded single- crystal material layer and a composite substrate with the further growth of a wide bandgap material layer on the surface of the thin wafer bonded single-crystal material layer.
Another object of this invention is structures and methods to make a composite substrate with improved thermal conduction, improved thermal expansion, improved microwave insulating characteristics, improved electrical conductivity and /or improved optical transmission properties, to which a single crystal material layer is direct wafer bonded and on which at least one epitaxial layer is grown.
Another object of this invention is the methods to laterally isolate regions of the composite substrate so that stresses generated in one polycrystalline substrate layer due to differences in thermal expansion between the layers of a composite substrate do not cause excessive stress in the single-crystal layer that is direct wafer bonded to the composite substrate and the epitaxial layer that is grown on the single-crystal layer.
These and additional objects of the invention are accomplished by the structures and processes hereinafter described which incorporate a composite substrate, an epitaxial layer and a single crystal film between the substrate and the epitaxial layer.